The thickness of paper is decreased with lighter weight of paper, high-speed papermaking and incorporation of higher amount of deinked pulp for the purpose of reducing burden on the environment and reducing transport costs in recent years. While bulky paper is desired, the stiffness of paper is proportional with the cube of thickness, and thus a decrease in the thickness of paper causes a reduction in stiffness.
The stiffness of paper exerts significant influence on feeling of high qualities, operativeness in papermaking and printing, the durability of a box, etc., and a reduction in stiffness gives a feeling of low qualities and causes paper clogging in operation and swelling of a box etc.
Techniques of improving stiffness include a method (1) wherein unit pulp amount (basis weight) is increased, a method (2) wherein a paper-strengthening agent or the like is used, etc., but there is a problem that in the method (1), the amount of necessary pulp is increased and the weight of paper is increased, and in the method (2), paper strength (difficulty in tearing paper) is improved and stiffness is somewhat improved, but satisfactory levels are not reached.
JP-A 8-170296 discloses a paper internal agent comprising fine polymer particles of vinyl monomers or diene monomers with a dispersant cationic polyvinyl alcohol having a mercapto group, and JP-A 11-302992 discloses an additive for papermaking, which is based on grafted starch obtained by graft-copolymerizing (meth)acrylamide-containing monomers while maintaining the form of starch particles without gelatinizing starch, and these preparations improve stiffness to a certain extent, but are still unsatisfactory.
With respect to techniques of improving the bulk of paper, Japanese Patent No. 2971447 discloses a polyvalent alcohol/fatty acid ester compound, and Japanese Patent No. 3283248 discloses a paper quality improver having a water freeness of 4% or more and improving at least two items selected from bulk, whiteness and opaqueness, but there is also demand for improvement of stiffness.